1. Field of the Invention
The present invention relates to a wireless communication system for mutual communication among a plurality of wireless stations such as a wireless LAN (Local Area Network), a wireless communication apparatus, a wireless communication method and a computer program, and more particularly to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program, in which a wireless network is configured by Ad-hoc communication without relationship between a controlling station and a controlled station.
2. Description of Related Art
More in detail, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program, in which a self-organized distribution type wireless network is formed in a communication environment preparing a plurality of channels, without interference between neighboring wireless systems and without relationship between a controlling station and a controlled station, and more particularly to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program, in which each communication station suitably determines its own beacon transmission channel to form a self-organized distribution type multi-channel wireless network.
By configuring a LAN by interconnecting a plurality of computers, information such as files and data and peripheral machines such as a printer can be shared, and information exchange such as transfer of e-mails and data content can be made.
Conventionally, a wired LAN connection is generally realized by optical fibers, coaxial cables or twist pair cables. However, in this case, line laying works are necessary, it is difficult to configure a network with ease, and a cable distribution becomes complicated. The motion range of apparatuses after the LAN configuration is limited by cable lengths, which is inconvenient.
A wireless LAN has drawn attention as a system releasing a user from LAN wiring of a wired system. According to a wireless LAN, most of wired cables can be omitted in a work space such as an office so that communication terminals such as personal computers can be moved relatively easily.
In recent years, demands for a wireless LAN system are increasing considerably because of its high speed and low cost. Introduction of a personal area network (PAN) has been studied recently in order to perform information communication by configuring a small scale wireless network among a plurality of electronic machines existing about each person. Different communication systems have been stipulated by utilizing frequency bands unnecessary for licenses by supervisory offices, such as a 2.4 GHz band and a 5 GHz band.
As one of the standard specifications of wireless networks, IEEE (The Institute of Electrical and Electronics Engineers) 802.11 (e.g., refer to Non-patent Document 1), HiperLAN/2 (e.g., refer to Non-patent Document 2 or Non-patent Document 3), IEEE 802.15.3, Bluetooth communication and the like can be enumerated. The IEEE 802.11 standard has various wireless communication schemes such as the IEEE 802.11a standard and the IEEE 802.11b standard depending upon a difference of a wireless communication scheme and a frequency band in use.
In order to configure a local area network by using wireless technologies, a method is generally used by which one apparatus to be used as a control station called an “access point” or a “coordinator” is installed in an area and a network is formed under the collective control by the control station.
When information is transmitted from some communication apparatus in a wireless network having distributed access points, an access control method based on band reservation has been adopted widely by which a band necessary for transmitting the information is first reserved at an access point to use a transmission path without collision of information transmission with other communication apparatuses. Namely, synchronous wireless communication is performed by mutually synchronizing with communication apparatuses in the wireless network by distributing access points.
In a case where asynchronous communication is to be performed between communication apparatuses on the transmission side and reception side in a wireless communication system having access points, this wireless communication requires by all means wireless communication via an access point so that there arises the problem that a transmission path use efficiency is decreased.
As another method of configuring a wireless network, “Ad-hoc communication” has been devised in which terminals perform wireless communication directly and asynchronously. It can be considered that the ad hoc communication in which arbitrary terminals perform wireless communication directly without using a particular access point is suitable particularly for a small scale wireless network configured by a relatively small number of clients positioned near each other.
In a work environment in which information machines such as personal computers (PC) are prevailing and a number of apparatuses are mixedly used in an office, it can be supposed that a plurality of networks are configured in a superposed manner with scattered communication stations. In this state, if the wireless network uses a single channel, there is no room of recovering the situations that another system intrudes during communication and that the communication quality is degraded by interference or the like.
To avoid this, a conventional wireless network system generally adopts a method by which a plurality of frequency channels are prepared for coexistence of other networks and a communication operation starts by making a wireless communication apparatus serving as an access point select one frequency channel.
The multi-channel communication scheme of this type can maintain a network operation and realize coexistence of other networks by switching a frequency channel to be used, when another system intrudes during communication or a communication quality is degraded by interference or the like.
For example, a high speed PAN system of IEEE 802.15.3 also prepares a plurality of frequency channels usable by the system and adopts the algorithm that after a power is turned on, a wireless communication device selects a usable frequency channel by executing a scan operation for all usable channels in order to confirm whether or not there are devices which are transmitting a beacon signal as the Piconet Coordinator (PNC) around the wireless communication device.
In an ad hoc network of a self-organized distribution type without relationship between a controlling station and a controlled station, resource management of frequency channels is important in order to suppress as much as possible interference with nearby different wireless networks under operation. However, in order to change frequency channels used in the network at a time, a representative station called a coordinator or an access point is required to instructs a use channel to each terminal station. In other words, it is difficult to switch a frequency channel in the ad hoc network.
In HiperLAN/2, for example, a method of changing frequency channels at a time can be considered in order to selectively use a plurality of channels. For example, an AP (base station) as a central control station repetitively notifies a frequency channel change, and, at some timing, AP and an MT (mobile station) connected to AP switch the channels at a time. A judgment whether the channel is switched or not is determined initiatively by AP. Information to be used for the judgment is collected by following a process procedure.
Specifically, the process includes the following procedures: (1) upon an instruction from AP, the connected MT temporarily suspends communication, scans other frequency channels to evaluate the channel quality, and sends the results to AP; (2) upon an instruction from AP, AP temporarily stops the transmission on a broadcast channel, and the connected MT scans the frequency channel in present use, evaluates the channel quality and reports the results to AP.
Bluetooth communication adopts a method by which a central control station called a master serving as a standard performs random frequency hopping to utilize squarely each frequency channel. Existence of the central control station called the master is essential for the network configuration and the central control station is used as the standard of a frequency channel hopping pattern and synchronization of a time axis direction. If the master extinguishes, the network formed until then is once disconnected so that a process of selecting a new master is necessary.
Also in a wireless LAN system of the IEEE 802.11 series, since a network is formed by using the frequency channel initially set by an access point, it is difficult to configure an ad hoc network without disposing a base station. When communication with a wireless communication apparatus (terminal) covered by AP operating at another frequency channel is to be performed, it is necessary to connect APs by wired LAN cables. Namely, if covering APs are not connected, communication is not possible even if the wireless communication apparatuses (terminals) physically existing adjacent to each other are covered by different APs.
Also in a high speed wireless PAN system of IEEE 802.15.3, although it is possible to initially scan all frequency channels and search a neighbor coordinator, if an operation starts once at a particular frequency channel, it is not possible to grasp the use state of other frequency channels. Therefore, even if a neighbor Piconet using a different frequency channel exists, communication with a wireless communication connected to the Piconet is impossible.
As above, the conventional methods require a complicated mechanism such as timings of frequency channel switching, a setup process to be realized by message exchange for starting a frequency channel switching operation through mutual synchronization of participating terminals, and an adjustment process to be used for determining frequency channel switching. It is also essential that a central control station initiatively performing control exists, such as AP in IEEE 802.11 and HiperLAN/2 and a master in Bluetooth communication. If the central control station such as AP and a master extinguishes, some protocol process of selecting a substitute central control station or a manual setting change work is necessary, resulting in the problem that communication is intercepted during this process.
A wireless communication system has been proposed which determines a frequency channel by measuring not only interference of own channel but also interference of adjacent channels by using these channels (e.g., refer to Patent Document 1), this system realizing a multi-channel with involvement of a base station.
For example, a method in which a communication station specifies a traffic reception channel by transmitting beacons through the optimum channel for the communication station itself, that is, the local station, can be considered. However, there is a possibility that, even when the channel is the optimum channel for the local station, the channel is one under interference for a communication station receiving the beacons. For example, when a beacon transmission channel of one station is an interference channel of the other station or an unusable channel having deteriorated communication quality, these communication stations fall into a state of a deadlock in which the communication stations cannot eternally recognize mutual existence, though the communication stations can perform communication with each other through the other channels.
[Patent Document 1]
    Japanese Patent Application Publication Hei 6-37762[Non-Patent Document 1]    International Standard ISO/IEC 8802-11:1999(E) ANSI/IEEE Std 802.11, 1999 Edition, Part II: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications[Non-Patent Document 2]    ETSI Standard ETSI TS 101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions[Non-Patent Document 3]    ETSI Standard ETSI TS 101-761-2 V1.3.1 Broadband Radio Access Network (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 2: Radio Link Control (RLC) sublayer
An object of the present invention is to provide an excellent wireless communication system, wireless communication apparatus, wireless communication method and computer program, which can properly configure a suitable ad hoc network without any interference between communication stations in a communication environment where a plurality of channels are prepared.
Another object of the present invention is to provide an excellent wireless communication system, wireless communication apparatus, wireless communication method and computer program, which can perform a channel access by effectively utilizing a plurality of frequency channels in a wireless network of a self-organized distribution type without relationship between a controlling station and a controlled station.
A further object of the present invention is to provide an excellent wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program, all capable of evading a deadlocked state, in which each communication station cannot recognize mutual existence, and capable of forming a self-organized distribution type multi-channel wireless network.
The present invention has been made in consideration of the above-described issues, and a first aspect of the present invention provides a wireless communication system for forming a network by a plurality of wireless communication stations without disposing a control station, in which each communication station transmits a beacon at a predetermined beacon period through its own beacon transmission channel, and transmits an interference annunciation signal describing information related to a channel under interference of a self station through another usable channel at a predetermined period.
The “system” used in this specification means a logical collection of a plurality of apparatuses (or functional modules realizing specific functions) and does not specifically refer to whether each apparatus or function module is accommodated in a single housing.
In the self-organized distribution type wireless communication system, each communication station notifies beacon information in the transmission frame period and executes a scan operation for beacon signals from other stations to recognize the network configuration. In a self-organized distribution type network using multi channels, the transmission frames are multiplexed on the frequency axis as many as the number of usable channels. Therefore, a communication station cannot receive a beacon unless it moves to the same channel as that of another communication, at the beacon transmission timing, resulting in that a new entry station is difficult to determine its own beacon transmission timing and transmission channel.
Moreover, there is the possibility that a channel is an optimum one for a communication station but the channel is one under interference for another station as a communications partner. For example, when a beacon transmission channel of one station is an interference channel or an unusable channel having deteriorated communication quality for the other station, these communication stations fall into a state of a deadlock in which the communication stations cannot eternally recognize mutual existence, though the communication stations can perform communication with each other through the other channels.
Accordingly, the present invention considers the following matters. That is, each communication station periodically transmits a beacon signal by means of a channel having the smallest degree of interference for the local station among a plurality of frequency channels. Moreover, in some interference situations, a beacon transmission channel selected in accordance with a criterion for the local station is not always a channel capable of being received by all neighbor communication stations. In specific, a communication station under interference from another system is made to transmit a particular signal called as interference annunciation signal periodically through all of the channels under no interference to inform neighbor stations of the existence of the local station and of the channel under interference.
Moreover, a neighbor communication station which has received the interference annunciation signal from another station compares its own beacon transmission channel with interference information of the interference annunciation signal. Then, when it becomes clear that a beacon is transmitted through a channel whose interference level is high, the neighbor communication station changes its beacon transmission channel. Similarly, also at the time of receiving a beacon, the neighbor communication station monitors an interference situation at any time, and selects a channel through which it can easily hear a beacon from a communication station which wants to perform communication frequently. That is, the transmission of beacons is to be suppressed in a channel under interference to a degree at which the beacons cannot be heard.
Consequently, according to the present invention, each communication station can determine a communication channel to evade interference efficiently in a self-organized distributed manner, and further can considerably improve communication capability by utilizing a plurality of channels effectively.
Moreover, a communication station is made to perform a channel scanning operation of each channel at a period which does not synchronize with the transmission period of an interference annunciation signal.
The channel scanning operation enables to discover a new beacon of a station newly entering the network and to detect secession of a beacon of a station existed in the network. Moreover, by performing the channel scanning at a period which does not synchronize with the transmission period of interference annunciation signals, it becomes possible to receive an interference annunciation signal IAS from a neighbor station surely at a frequency of once every several times at the time of channel scanning. Consequently, even if communication stations are in a state of being impossible of receiving mutual beacons owing to interference, receiving the interference annunciation signals periodically enables to evade a deadlock.
Moreover, a communication station may be configured not to transmit any interference annunciation signals when there are no channels under interference. By omitting transmission of unnecessary interference annunciation signals, the availability of transmission paths can be improved, and furthermore the power consumption of the apparatus can be decreased.
A communication station transmits an interference annunciation signal at a period sufficiently longer than a beacon interval. Each communication station can discover a new beacon of a station newly entering the network and detect secession of a beacon of a station existed in the network by performing a multi-channel scanning operation at a period which does not synchronize with the transmission period of interference annunciation signals. Moreover, by performing channel scanning at a period which does not synchronize with the transmission period of interference annunciation signals, a communication station can surely receive an interference annunciation signal IAS from neighbor stations at the frequency of once every several times at the time of channel scanning.
Now, when communication through a specific channel is difficult owing to the interference from a certain strong interference source, it is expected that the possibility that a neighbor terminal is also under similar interference is high. As a result, on a condition of the existence of interference, it is conceivable that a packet informing the interference needlessly increases.
Moreover, the packet informing interference is not utilized unless a change such as an entry of a new terminal from another network is not produced. In other words, it is ineffective to transmit interference annunciation signals at a high frequency even in a steady state. Moreover, because a communication station cannot perform data transmission during a period in which another station transmits the interference annunciation signals, the high frequency transmission is an overhead of the system.
Accordingly, when it becomes clear that neighbor terminals having the same interference exist elsewhere as a result of an examination of neighbor terminals, the communication station changes the transmission frequency of the interference annunciation signals to transmit them at a thinned-out frequency according to the number of the neighbor terminals having the same interference. As a result, although the transmission sphere of the packets informing the interference of each communication station is somewhat shifted, the packets are informed at the same frequency on an average as the whole system.
Moreover, when a new entry station appears in a wireless system, there is the possibility that a phenomenon in which the networks themselves overlap each other has been generated in a case of the appearance of the new entry communication station from another network. In such a case, for notifying the new entry station of a network configuration and interference channel information at an early stage, the existing communication station is required to transmit interference annunciation signals at a frequency equal to or more than a predetermined value. Accordingly, the communication station makes the possibility of the new entry station's discovery of the interference annunciation signals higher by increasing the transmission frequency of the interference annunciation signal for a predetermined time period (for example, by setting the transmission frequency to be same as a beacon interval) in response to the new entry of the communication station from the other network. Moreover, when the communication station detects a new entry station, the communication station informs neighbor stations of the existence of the new entry station, and increases the transmission frequency of the interference annunciation signal only for a predetermined time period in response to the reception of such information.
Moreover, when a station enters a wireless system, for early information of a network configuration and interference channel information, it is necessary to transmit an interference annunciation signal at a frequency equal to or more than a predetermined value. However, after the completion of the entering operation, the necessity of the information of interference is gradually decreasing. Accordingly, the communication station may gradually decrease the transmission frequency of interference annunciation signals from an initial value according to the elapsed time from the entering of the new communication station.
Moreover, a second aspect of the present invention is a computer program described in a computer readable format for executing wireless communication processing on a computer system for operating in a self-organized distributed manner under a wireless communication environment, the program characterized by comprising: an interference annunciation signal generation step for generating a beacon signal and an interference annunciation signal, both describing information related to a channel under interference of a local station; an interference annunciation signal analysis step for analyzing the beacon signal and the interference annunciation signal, both received from a neighbor station; a channel setting step for setting a data transmission and reception channel; and a communication control step for controlling a transmission operation of the beacon signal and the interference annunciation signal of the local station, and a reception operation of the beacon signal and the interference annunciation signal from the neighbor station.
The computer program according to the second aspect of the present invention is one defining the computer program described in the computer readable format for implementing predetermined processing on the computer system. In other words, by installing the computer program according to the second aspect of the present invention into the computer system, a cooperative operation is exhibited on the computer system, and the computer system operates as a wireless communication apparatus. By constituting a wireless network by starting a plurality of such wireless communication apparatus, similar operations and advantages as those of the wireless communication system according to the first aspect of the present invention can be obtained.
According to the present invention, an excellent wireless communication system, wireless communication apparatus, wireless communication method and computer program can be provided which can configure reliably a proper ad hoc network without any interference between communication stations in a communication environment preparing a plurality of channels.
Further, according to the present invention, an excellent wireless communication system, wireless communication apparatus, wireless communication method and computer program can be provided which can perform a channel access by effectively using a plurality of frequency channels in a self-organized distribution wireless network without relationship between a controlling station and a controlled station.
Moreover, according to the present invention, it is possible to provide an excellent wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program, all capable of evading a deadlocked state in which each communication station cannot recognize mutual existence, and capable of forming a self-organized distribution type multi-channel wireless network.
Moreover, according to the present invention, in a self-organized distribution type multi-channel wireless network, each communication station efficiently performs frequency arrangement, and thereby the throughput of the whole system is improved, and further the influences to neighbor other wireless systems can be decreased.
Other objects, features and advantages of the present invention will become apparent from the preferred embodiments of the present invention to be described later and the detailed description given in conjunction with the accompanying drawings.